A receiving device of radio communication has a high frequency circuit that has: a mixer which down-converts a high frequency receive signal received by an antenna into an intermediate frequency or a base band; and a low-pass filter (LPF) which cuts off a high frequency component from an output signal of the mixer. The receive signal down-converted by the high frequency circuit is converted into a digital receive signal, and the digital receive signal is demodulated and error-corrected by a digital base band circuit.
For radio communication, high-speed data communication and communication with many terminals are demanded. Therefore a bandwidth allocated to one terminal may be changed according to a number of terminals. For example, if a number of terminals increases, a bandwidth allocated to each terminal is decreased so that many terminals communicate, and if a number of terminals decreases, a bandwidth allocated to each terminal is increased so that high-speed data communication is enabled.
Accordingly an LPF preferably variably-controls the cut-off frequency. For an LPF, an RC filter, and a GmC filter (transconductance and capacitance filter) or the like are widely used. The RC filter variably-controls the cut-off frequency by variably-controlling the cut-off frequency by variably controlling a feedback resistor or a feedback capacitor of an operational amplifier. The GmC filter has an OTA (Operational Transconductance Amplifier) which variably-controls the Gm (transconductance) instead of the input resistor and feedback resistor of the RC filter, and variably-controls the cut-off frequency by variably-controlling the Gm or the feedback capacitor of the OTA.
Known arts are Japanese Laid-open Patents 2005-295460, 2000-341089 and H08-139607.
In order to variably-control the cut-off frequency in the GmC filter, the Gm of the OTA is variably-controlled or a capacitance value of the feedback capacitor is variably-controlled. For this, many OTAs are provided, and a number of OTAs to be used is increased/decreased by a switch. Or many feedback capacitors are provided, and a number of capacitors to be used is increased/decreased by a switch.
However in either case, many OTAs and capacitors are formed if a high resolution of variable control is demanded, and therefore the (chip) area of the LSI increases. This means that if the GmC filter is created in a limited (chip) area, the resolution for variably-controlling the cut-off frequency decreases.